1. Field of the Invention
The present invention relates to a method for measuring an amount of bend of a moving mirror which is mounted on, for example, a wafer stage for carrying out coordinate measurement by a laser interferometric measuring method in an exposure system in which a reticle pattern is photolithographically transferred onto a wafer placed on the wafer stage. More particularly, the present invention relates to a moving mirror bend measuring method which is suitably applied to measure an amount of bend of a moving mirror in an exposure system of the type in which yawing of a wafer stage is corrected by rotating a reticle.
2. Related Background Art
To produce semiconductor devices, liquid crystal display devices, etc. by photolithography process, an exposure system is used in which a pattern formed on a reticle as a mask is photolithographically transferred onto each shot region on a wafer (or a glass plate or the like) coated with a photosensitive material. As an exposure system of this type, a reduction projection type exposure system (i.e. stepper) is frequently used in which a reticle pattern is sequentially transferred onto each shot region on a wafer placed on a wafer stage by stepwisely moving (stepping) the wafer stage according to the step-and-repeat method.
In such an exposure system, the coordinate position of the wafer stage in two directions (assumed to be directions X and Y) which perpendicularly intersect each other is generally measured by using two moving mirrors which are fixed on the wafer stage such that their reflecting surfaces perpendicularly intersect each other, and two laser interferometric measuring devices (hereinafter referred to as "interferometers") for two orthogonal axes which are disposed in association with the moving mirrors. In this case, it is desirable that the reflecting surfaces of the two moving mirrors should be perfect planes. In actual practice, however, the reflecting surfaces of the moving mirrors have unavoidable bend. It should be noted that unavoidable orthogonality deviation of the two moving mirrors is average bend of one moving mirror when the other moving mirror is defined as a reference. Therefore, orthogonality deviation is also treated as bend in the following description.
Accordingly, such control has heretofore been carried out that an amount of bend of each of the two moving mirrors (to be precise, an amount of bend of the reflecting surface thereof) has been measured in advance, coordinate values obtained by the interferometers are corrected by a software process on the basis of the result of the above measurement, thereby allowing the wafer stage to be accurately driven in two directions perpendicularly intersecting each other.
As a conventional method of measuring an amount of bend of a moving mirror, so-called vernier evaluation method is known. In the vernier evaluation method, a reticle formed with a plurality of measurement marks is used. First, a reticle pattern image is formed by exposure at a first position in the first row on the wafer. Then, the wafer stage is stepped in the direction X, for example, and a reticle pattern image is formed by exposure at a second position where an X-direction end of the image overlaps the first reticle pattern image. In the overlap region, a measurement mark image (main scale) formed by the first exposure and a measurement mark image (vernier) formed by the second exposure are disposed in close proximity to each other. The positional relationship between the main scale and the vernier in a state where the moving mirror has no bend has previously been known. Therefore, an amount of positional displacement between the main scale and the vernier relative to the design positional relationship is measured, thereby obtaining an amount of bend of the moving mirror. This is the principle of the vernier evaluation method.
Further, a conventional exposure system has an interferometer for measuring a yawing (rotation error) in the XY-plane of the wafer stage, which is installed separately from the pair of interferometers for coordinate measurement. The interferometer for rotation error measurement is installed in parallel to the interferometer for the Y-axis, for example, so as to apply a laser beam to the moving mirror for the Y-axis. Thus, a rotation angle of the wafer stage is measured from the difference between a value measured by the interferometer for the Y-axis and a value measured by the interferometer for rotation error measurement. In the exposure system having such an interferometer for rotation error measurement, the detected rotation angle is corrected for the amount of bend of the moving mirror for the Y-axis, which has previously been obtained, and then the reticle is rotated so that the remaining rotation angle component is canceled, thereby preventing rotation angle misalignment between each shot region on the wafer and the reticle.
However, the above-described conventional technique is incapable of measuring bend of a moving mirror at a portion thereof which is outside the exposure range on the wafer stage because an amount of bend of the moving mirror is measured from the result of the actual exposure. In the conventional practice, the interferometer for rotation error measurement is adapted to apply a laser beam to the moving mirror for the Y-axis, for example, at a position which is displaced in the direction X relative to the laser beam emitted from the Y-axis interferometer for coordinate measurement. Therefore, if the laser beam from the interferometer for the Y-axis is incident on an end region of the exposure range on the associated moving mirror, the laser beam from the interferometer for rotation error measurement is incident on a region outside the exposure range on the associated moving mirror. Consequently, the laser beam from the interferometer for rotation error measurement is incident on a region on the moving mirror for which an amount of bend has not accurately been measured. Accordingly, yawing of the wafer stage cannot accurately be corrected.
The reason for the above is that yawing of the wafer stage and the amount of bend of the moving mirror cannot accurately be separated from each other at a region outside the exposure range.